In the operation of nuclear reactors, hollow zircaloy tubes filled with enriched uranium, known as fuel assemblies, are burned up inside the nuclear reactor core. It is customary to remove these fuel assemblies from the reactor after their energy has been depleted down to a predetermined level. Upon depletion and subsequent removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its subsequent packaging, transporting, and storing. Specifically, the SNF emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times subsequent to removal from the reactor core.
In defueling a nuclear reactor, it is common to remove the SNF from the reactor and place the SNF under water, in what is generally known as a spent fuel pool or pond storage. The pool water facilitates cooling of the SNF and provides adequate radiation shielding. The SNF is stored in the pool for a period long enough to allow the decay of heat and radiation to a sufficiently low level to allow the SNF to be transported with safety. However, because of safety, space, and economic concerns, use of the pool alone is not satisfactory where the SNF needs to be stored for any considerable length of time. Thus, when long-term storage of SNF is required, it is standard practice in the nuclear industry to store the SNF in a storage cask subsequent to the brief storage period in the spent fuel pool.
Storage casks have a cavity adapted to receive a canister of SNF and are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of SNF, size and weight are often secondary considerations (if considered at all). As a result, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh more than 100 tons and have a height greater than 15 ft. A common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problem is that storage casks are generally too large to be placed in spent fuel pools. Thus, in order to store SNF in a storage cask subsequent to being cooled in the pool, the SNF must be removed from the pool, placed in a staging area, prepared for dry-storage, and transported to a storage facility. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
As a result of the SNF's need for removal from the spent fuel pool and additional transportation to a storage cask, an open canister is typically submerged in the spent fuel pool. The SNF rods are then placed directly into the open canister while submerged in the water. However, even after sealing, the canister alone does not provide adequate containment of the SNF's radiation. A loaded canister cannot be removed or transported from the spent fuel pool without additional radiation shielding. Thus, apparatus that provide additional radiation shielding during the transport of the SNF is necessary. This additional radiation shielding is achieved by placing the SNF-loaded canisters in large cylindrical containers called transfer casks while still within the pool. Similar to storage casks, transfer casks have a cavity adapted to receive the canister of SNF and are designed to shield the environment from the radiation emitted by the SNF within.
In facilities utilizing transfer casks to transport canisters loaded with SNF, an empty canister is first placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the spent fuel pool. The SNF that has been removed from the reactor and placed in wet storage racks arrayed on the bottom of spent fuel pools is then placed within the canister. The loaded canister is fitted with its lid. This loading operation is performed under water using remotely operated tools for grappling, lifting and placing.
The loaded canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the SNF-loaded canister for long-term dry storage in a storage cask. Once prepared, the transfer cask is transferred from the staging area and set atop a storage cask for transfer of the SNF-loaded canister.
Due to the extremely dangerous neutrons and gamma photons emitted by the SNF, transfer casks are typically designed to be large cylindrical vessels equipped with thick walls to provide radiation shielding to personnel. As such, transfer casks are very heavy structures, often weighing over 75 tons. When loaded with SNF and water, the weight can exceed 120 tons.
To lift and position transfer casks, nuclear power plants are equipped with overhead cranes that can access the spent fuel pool and the plant equipment receiving areas. The plant's crane must have sufficient capacity to support the weight of the loaded transfer cask, have sufficient range to access both the plant's spent fuel pool, canister staging area, cask loading area, and equipment receiving area. The capacity of the crane depends on the plant's crane lift rating and the ability of the crane's supporting structure to bear the load.
Many older and smaller nuclear power plants do not have sufficient crane capacity to lift and position larger transfer casks that have been developed. The process of upgrading the crane to a higher capacity is hindered by building structural limitations. Moreover, older power plants' supporting structures are often of unknown structural capability or are fabricated from materials that may not have the structural properties necessary to meet current safety requirements for lifting nuclear materials.
Many of the older plants have been shut down and possess a relatively few number of spent fuel assemblies so the cost of providing an upgraded crane and improved supports cannot be financially justified.
Cask lifting devices must have the ability to be able to strategically place the load-bearing members over high strength locations in the building and utilize combinations of specialized lifting components that do not interfere with the plant's existing fuel handling systems and crane. The device must provide adequate protection against such things as power failure, uncontrolled lowering of the load under a postulated failure of a single component, and uncontrolled lowering of the load under earthquake conditions. Specialty devices which provide protection against uncontrolled lowering of the load require large components that are expensive, difficult to install, and interfere with the existing plant structures, systems and components.